StringBuffer and StringBuilder

• String is immutable, StringBuffer/StringBuilder is mutable.

  每次對String進行修改都會產生新的物件，若經常改變同一個字串，最好不要用String。

    public class StringImmutableTest {
        public static void main(String[] args) {
            String str = new String("hello");
            str.concat(" world"); //會產生新的字串(hello world)，但沒被引用
            System.out.println("String: " + str);
  
            StringBuffer sb = new StringBuffer("hello");
            sb.append(" world");
            System.out.println("StringBuffer: " + sb);
        }
    }
  

    String: hello
    StringBuffer: hello world
  

• StringBuffer is synchronized(thread safe). StringBuilder is non-synchronized, so it faster than StringBuffer.

  確定不會在多執緒下執行，用StringBuilder以獲得較好的效能

• 使用+來串接字串時，編輯器會做最佳化:

  • String s = "a" + "b" + " c"; → String s = "abc"
  • String s = s1 + s2; → String s = (new StringBuffer()).append(s1).append(s2).toString();

  • 但如果在迴圈內串接字串，還是要改用StringBuilder，才能確保不會一直產生新的物件。

    String out = "";
    for( int i = 0; i < 10000 ; i++ ) {
      out = out + i;
      // => out = new StringBuilder().append(out).append(i).toString();
    }
    return out;
    

    StringBuilder out = new StringBuilder();
    for( int i = 0 ; i < 10000; i++ ) {
      out.append(i);
    }
    return out.toString();
    

• 其他常用方法

    public class StringBuilderTest {
      public static void main(String[] args) {
        System.out.println(new StringBuilder("hello").append("world"));
        System.out.println(new StringBuilder("hello").length());
        System.out.println(new StringBuilder("hello").reverse());
        System.out.println(new StringBuilder("hello").insert(5, "world"));
        System.out.println(new StringBuilder("hello").delete(1, 3));
        System.out.println(new StringBuilder("hello").substring(0, 2));
      }
    }
  

  helloworld
  5
  olleh
  helloworld
  hlo
  he
  

Reference

[ Java 文章收集 ] 是 String ， StringBuffer 還是 StringBuilder

Java: String concat vs StringBuilder - optimised, so what should I do?